Light weight structural elements and extended surface



J. R. GIER, JR

Nov. 3, 1959 LIGHT WEIGHT STRUCTURAL ELEMENTS AND EXTENDED SURFACE Filed Jan. 10, 1955 2 Sheets-Sheet l Nov. 3, 1959 J. R. GIER, JR

LIGHT WEIGHT STRUCTURAL ELEMENTS AND EXTENDED SURFACE Filed Jan. 10, 1955 2 Sheets-Sheet 2 INVENTOR. yvvQ' n/z BY wfmeA/l m LIGHT WEIGHT STRUCTURAL ELEMENTS AND EXTENDED SURFACE .l'ohn R. Gier, In, Cleveland, Ohio Application January 10, 1955, Serial Ne. 480,668 5 Claims. 01. 257256) This invention relates to composite light Weight structural elements for use as light weight decking and the like, and to strip fin extended surfaces for use as heat exchangers and the like, and to a method of fabricating the same.

For the purposes of illustration, the article and method are described hereinafter as applied to heat exchangers of the plate type in which strip fins of metal are disposed between adjacent metal plates or sheets and bridge the space therebetween, the application of the invention to other articles being readily apparent from the illustrative examples.

In the design of a compact, extended surface multilayered heat exchanger of the plate type, it is generally desirable to balance the heat transfer characteristics of the overall fluid passage of each layer against the overall fluid passage of adjacent layers.

When the two fluids have unequal heat transfer characteristics due to ditferences in density, composition, and the like, it is necessary, in order to achieve the desired balance, to passages differing in film coefficient, in area, in hydraulic diameter, and in general configuration.

For example, in plate type regenerators for gas turbines, the ideal extended surface comprises streamlined pin fins having a high film coefficient in relation to their low friction factor on the low pressure side balanced by a lower film coefficient, a smaller area, and a smaller hydraulic diameter, on the high pressure side.

In my United States Letters Patent No. 2,678,808, there is disclosed a structure made up of plates with streamlined pin fins disposed therebetween and bonded thereto, and in my copending application, Serial No. 480,669, filed of even date herewith and entitled Method of Making Light Weight Structural Elements, there is disclosed a plate type exchanger comprised of alternate layers of pin fins and strip fins.

Many attempts have been made to provide effective strip fin layers for such exchangers. Examples include extended surfaces in which channels of thin metal sheet or corrugated metal sheets bonded between plates are used for fins. This type of fins has a number of functional and fabricating disadvantages. For example, the bends in a channel at the junctures of its base with the sides are at the points of attachment of the channel with one of the plates. These bends adversely lengthen the path of flow of heat through the fin from one plate to the other, the resultant heat flow path being longer than the space between the plates. Furthermore, the stress due to hydrostatic pressures concentrate at these bends and tends to distort the channels into circular cross section, thus initiating ruptures of the joints due to a peeling action. Again, the bends define with the attached plate exteriorly of the channel wedge shaped pockets which are coextensive endwise with the channels. These pockets are not effective for heat transfer due to the stagnation of the fluid therein. Further, they form Zones which can readily be fouled by the accumulation of dirt and foreign matter.

rates Patent Fins in the form of corrugated sheets with the crests of the corrugations engaging the plates which separate the layers are open to the same objections aggravated by the fact that not one separating plate but both separating plates are connected in the manner described to each fin.

Furthermore, both channels and sheets of corrugated metal introduce an extra thickness of metal overlying the plates between adjacent fins. Again, they require expensive tooling to make them to the accuracy and uniformity of height required when a multi-layer assem bly of plates and fins is to be made. A complete set of different tools is required for any change in fin height or spacing.

Finally, fins of corrugated and channel shaped cross section tend to compress and deform under the compressive loads and temperatures required for brazing together multi-layer assemblies of extended surfaces.

The present invention relates primarily to the manufacture of the strip fins assemblages or grids which are subsequently bonded between the adjacent plates in the manner disclosed in my above identified copending application.

In accordance with the present invention, the difiiculties common to the channels and corrugated sheets are minimized by providing a number of separate individual strip fins of thin sheet metal and of rectangular cross section assembled in a row in fiatwise spaced relation to each other so as to provide a grid in which the opposite edge surfaces of each strip are exposed at, and lie in, the opposite faces of the grid, respectively, and in which the fins are bonded to plates by their edges instead of by their flat faces or by marginal flanges and the like, such as would be the case in connection with channel shape fins and fins formed of corrugated sheet.

Further, in accordance with the present invention, the strips or fins are arranged in a grid in which they are held in fiatwise spaced and parallel relation to each other by cross tie members.

Each cross tie member is relatively rigid in lengths equal to the space between the fins and extends endwise of the row of fins. The cross tie members are spaced from each other lengthwise of the fins. They are bonded to the fins so that none extends outwardly, in a direction edgewise of the fins, appreciably beyond the plane of the associated face of the resultant grid. Each of the cross tie members is of small dimension in a direction edgewise of the fins and therefore does not deleteriously affect or obstruct the flow of fluid endwise of the fins in the space between the fins. The cross tie members preferably are Wires of small diameter relative to the thickness of the fins. When the grids are installed between plates, the cross tie members lie on the surface of the plates and protrude radially of the members into the spaces between the fins to only such a slight distance that not only do they not materially obstruct the flow of fluid between the fins but break up the laminar layer and thus increase the heat exchange between the fluid and plate surface.

This requires, of course, permanent cross-tie members, that is, cross-tie members capable of withstanding brazing temperatures so that they can remain a part of the grid when it is incorporated in other structures as, for example, between plates, Further, as mentioned, each cross -tie member is relatively rigid in lengths equal to the space between the fins whereas in its full length before connection to the strips it is readily bendable, as is apparent from its size as indicated in the drawings.

In addition, the fins are configured on the surface either by notching or indentation, or perforation so as to reduce the laminar effect and increase the heat exchange without greatly increasing the frictional resistance to flow.

Various objects and advantages will become apparent from the following description wherein reference is made to the drawings, in which:

Figure 1 is a top plan view partly in section, illustrating sub-assembly or grid of fins arranged between the confining plates;

Figures 2 and 3 are a front elevationand a right end elevation, respectively, of the structural element illustrated in Figure l;

Figure 4 is an enlarged fragmentary top plan view, partly in section, of a part of the structural element illustrated in Figure l and showing the relation of the cross tie members and the strip fins;

Figure 5 is a vertical sectional View taken on the line 55 of Figure 4;

Figure 6 is a fragmentary right end elevation of the structure illustrated in Figure 4;

Figures 7 and 8 are an enlarged top plan view and an enlarged front elevation, respectively, of one of the strip fins used in the structure of Figures 1 through 6, showing the means for breaking up the laminar layer of the fluid flowing between the fins;

Figure 9 is an enlarged fragmentary front elevation of one of the strip fins showing a modified form of means for breaking up the boundary or laminar layer;

Figures 10 and ll are fragmentary horizontal and vertical sectional views, respectively, taken on lines 1tlll0 and l111 of Figure 9;

Figure 12 is a perspective view, partly in section, of a modified fin assemblage bonded between confining plates;

Figure 13 is a schematic top plan view of an apparatus for forming the fin grid of the present invention; and

Figure 14 is a front elevation of the apparatus illustrated in Figure 7.

Referring first to Figure 1, the invention is shown incorporated in a single layer extended surface structure or article, indicated generally at 1, having a top plate 2, a bottom plate 3, and a plurality of strip fins 4 disposed therebetween. In the form illustrated, the plates 2 and 3 are planar and parallel to each other and the fins 4 are of uniform height and thickness. The fins preferably are formed by shearing a wide strip of thin metal into a plurality of relatively narrow ribbons. The fins 4 extend at abrupt angles, and preferably 90, to the surfaces of the plates 2 and 3 and are bonded thereto, preferably by brazing in the manner set forth in my copending application Serial No. 480,669, filed of even date herewith. I

The fins 4 are arranged in flatwise spaced and parallel relation to each other. They are disposed between the plates 2 and 3 with their edges in substantially direct contact with the plates, any spacing of surface of the edges from the plates not exceeding that of the thickness of capillary films, of such brazing material as may be used in brazing the fins to the plates, which can be drawn in between the edges and the plates by capillarity. The lateral faces of the fins 4 do not come in contact with the plates 2 and 3, nor do the fins have any marginal attaching flanges.

For convenience in handling the fins 4, they are secured together in assemblages each of which comprises a grid formed of a plurality of fins 4 arranged in a row in flatwise spaced relation to each other and with their opposite edges exposed at the opposite faces of the grid, respectively. In most instances the grids are placed between fiat plates and hence the faces of each grid define parallel planes, respectively. If the grids are to be disposed between curved surfaces, the edges of each grid at one face thereof may define a curved surface such that when that face of the grid is placed against the curved surface to which it is to be bonded, the edges of the fins will be juxtaposed thereagainst. Thus, the fin contactwith its plates is at the exposed edge surfaces of the fins.

In the form illustrated, which is the type most customarily used, the fins are disposed between parallel planar plates.

In order to hold the fins assembled as a grid, so that they can be handled and positioned readily between the plates, the fins are tied together in their parallel spaced relationship by means of permanent cross tie members 5. The cross tie members 5 may be arranged at one or the other face of the assemblage of fins but preferably are arranged, especially when the fins are to be placed between parallel surfaces, at opposite faces of the assemblages. If the grid is to be flexed against a curved plate or surface so that the fins extend radially thereof, then cross tiemembers may be used at one face only.

The cross tie members 5 are spaced from each other longitudinally of the fins and each at one face of the grid preferably is offset endwise of the fins from the adjacent ones which are at the opposite face of the grid. The cross tie members 5 are preferably in the form of metal wires or strands which are bonded to the edges of the fins by welding accompanied by pressure which embeds the wires into the fins while the fins are softened at the points of contact, as best illustrated in Figures 2 and 5. Thus, that surface of each wire disposed outermost of the grid in a direction edgewise of the fins is substantially flush with the corresponding face of the grid, or,

at least does not extend outwardly beyond its corresponding grid surface a distance greater than the thickness of a capillary brazing film.

' The cross tie members 5 must be of such cross section that they are relatively stiff and resist buckling under endwise compression in lengths equal to the space between a pair of adjacent fins.

In general, soft metal wire which has a diameter no greater than the thickness of the sheet metal of which the fins are formed is used. This appears to be the best relation for causing the wires to embed in the fins when applied thereto and heatedand pressed against the fins by spot welding equipment.

The assemblage of the fins may-be placed between plates and bonded by brazing as pointed out in my above identified copending application.

Here it is to be noted that since the edges of the fins engage the plates there is a balanced stress on each fin where it is bonded to the plates, that is, very minute fillets of brazing material are deposited at the intersection of each face of a fin with its associated plate. This eliminates the peeling effect which occurs when fins of channel shape cross section are bonded to a plate. In the latter instance the peeling effect is due to the fact that the bonding material fills the small V-shaped area at the juncture.

at the base of the channel, and one of its flanges along one face of a given fin but not at the other face, the other face being connected only to the base of the channel.

As is clear from Figure 5, the permanent cross tie members 5 project radially slightly into the space between adjacent fins, thus causing a slight surface irregularity on the surface of the plate between the fins wherever the cross tie member is juxtaposed against the plate. These irregularities break up the laminar layer and improve the heat exchange relation.

In order to break up the laminar layer on thefaces of the fins, the fins may be provided at intervals along their length with notches or indentations 6 which extend from one edge to the other and preferably directly across the faces of the fins. These notches preferably are formed so that the surfaces of the fin adjacent thereto, indicated at 7, slope gradually inwardly from the outer plane of the face of the fin toward the base or deepest part of the associated notch.

Another means for breaking up laminar layer is illustrated in Figures 9 through 11. -As there illustrated,

holes or perforations are formed in the fins. For example, a fin 8, similar to the fins 4, is provided with a series of holes or perforations 9 extending therethrough. These holes preferably extend from near the top to near the bottom of the associated fin and are relatively narrow endwise of the fin. They are formed so that their Walls, as indicated at 10, slope inwardly on a gradual curve from the opposite faces of the associated fin 8, thus interrupting the boundary layer at minimum increase in flow friction along the fins. These passages thus are very effective in breaking up the laminar layer and assuring effective heat exchange.

Referring next to Figure 12, there is shown a modification of the fins in which the fins instead of being planar for their full length are slightly corrugated endwise but not transversely. They are assembled so that the crests of the corrugations face edgewise of the plates and not toward the plates. Thus, each fin, indicated at 12, presents a series of planes 12a angularly disposed relative to each other lengthwise of the fin. These fins are secured together in face to face parallel relation with the crests of each aligned with the corresponding crests of the others and are secured together by cross tie members in the same manner as the fins 4 were secured by the cross tie members 5. The purpose of the endwise corrugations is to break up the laminar layer periodically along each fin endwise of the fin at a minimum increase in flow friction.

In order to illustrate the method in which the fins are made there is shown schematically in Figures 13 and 14 an apparatus suitable for forming the grids which heretofore have been described. In this form, a sheet of metal 14 of a thickness corresponding to that desired in the finished fins is fed from a suitable reel 15 through suitable feed and levelling rolls indicated at 16 and thence through a press 17 which either indents or perforates the strip to provide the notches 6 or perforations 9. The stock thus indented or perforated passes through suitable slitting rolls 18 which cut the strip into a plurality of ribbons which become the fins 4, the strips at this location being disposed in edgewise relation to each other. These fins then pass through a suitable cut off shear 19 which cuts them to length and the cut off lengths are fed through suitable twisting guides 21) by which each fin is twisted from its edge to edge relation with the others to spaced face to face relation with the others wherein it rests on its edge, thus disposing the fins in parallel face to face spaced relation to each other. In this condition, the fins pass through suitable feed rolls 21 and guides 22 which hold them in the assembled face to face spaced relation as they progress toward the right in Figures 13 and 14. While held in this position, they pass through a resistance welding device 23 at which location the cross tie members 5 are fed from a suitable spool 24 across the assembled strips and out off by a cut off 25. The strips rest against an electrode 26 with the cross tie member 5 between the strips above the electrode. While in this position, they are engaged by a complementary electrode 27 thus heating them and the strips at their junctures and pressing against the tie member 5 and embedding them in the softened margins of the strip so that their outermost surface edgewise of the strip lie flush with the plane of the edges of the fins at one face of the assemblage.

At the same time, a member 5 is fed from a suitable spool 28 across the edges of the fins at the opposite face of the assemblage of fins, the wire being cut off to a length by a cut off 29 and it and the strips being engaged between the complementary electrode 30 and electrode 31. Thereby, the latter cross tie member is welded to the opposite face of the assemblage of the fins so that its outermost surface is flush with the outermost face of the assemblage. In the example shown, the fins have been cut to length prior to the welding operation and accordingly, the grid is discharged when welded. However, the fins can be cut to length after they have been connected together by the cross tie members. ()orrugated fins such as shown in Figure 12 are employed and may be corrugated at any time after slitting, even after welding the cross tie members thereto, if desired.

Any number of welders may be used depending upon the length of the fins and the spacing of the cross tie members, the final welder indicated at 32 being shown in Figures 13 and 14. The assemblage thus provided, without further preparation, is in condition to be assembled between the sheets to which it is to be attached, preferably by brazing as defined in my above identified copending application.

Having thus described my invention, I claim:

1. A grid structure for the purposes described and comprising a plurality of strips of thin sheet metal arranged in a row in flatwise spaced relation to each other and providing a grid in which one edge of each stn'p lies in one surface of the grid and the other edge of each strip lies in the other surface of the grid, permanent cross-tie members extending generally endwise of the row of strips and transversely of the individual strips and spaced from each other lengthwise of the strips at at least one of said surfaces, each of said cross-tie members bridging across a plurality of spaces between successive strips and being embedded in, and permanently bonded at its periphery to, the strips with its peripheral surface areas which are outermost in a direction edgewise of the strips substantially flush with the edges of the strips at said one of the surfaces of the grid, and its surface areas which are innermost lying close to said one of the surfaces of the grid, each cross-tie member being of small dimension edgewise of the strip and relatively rigid in lengths as long as the spacing of the strips and, before connection to the strips, being readily bendable in lengths as long as the total spacing of several strips.

2. A grid according to claim 1 wherein said dimension of each cross-tie member in a direction edgewise of the strips is at least as small as the thickness of a strip.

3. A grid structure according to claim 1 wherein the cross-tie members are wires of small diameter.

4. A grid according to claim 1 characterized in that said cross-tie members are provided at both surfaces of the grid.

5. A grid according to claim 1 characterized in that metal sheets having surfaces conforming in contour to the opposite faces of the grid, respectively, are juxtaposed against the conforming faces of the grid, respectively, the juxtaposed face of each sheet being brazed in face to face relation to the cross-tie members and the edge surfaces of the strips at its associated face of the grid.

References Cited in the file of this patent UNITED STATES PATENTS 2,108,795 Budd Feb. 22, 1938 2,303,369 Kleist Dec. 1, 1942 2,571,631 Trumpler Oct. 16, 1951 2,595,457 Holm et al. May 6, 1952 2,614,517 Peterson Oct. 21, 1952 2,670,936 Holm et al. Mar. 2, 1954 2,686,957 Koerper Aug. 24, 1954 

